The present exemplary embodiment relates to electrical switching mechanisms. It finds particular application in conjunction with medium voltage earthing switches, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
It is common to provide protection to technicians servicing an electrical component enclosure through the provision of an earthing switch. A typical earthing switch includes one or more blade contacts mounted on a rotatable shaft. An actuating mechanism rotates the shaft to move the blade contacts between an open position and a closed position in contact with a grounding electrode. The earthing switch is typically installed between a distribution bus and a circuit breaker connecting the distribution bus to a main line. The earthing switch, when closed, grounds the distribution bus.
Prior to earthing the line or bus terminals, it is typical to disconnect the upstream source of electrical power. In certain situations, however, the circuit may inadvertently be live during grounding. In other situations, the upstream source of electrical power may be inadvertently reenergized before performing closing of the switch. In still other situations, there could be back feed of electricity to the distribution bus such as, for example, in the case of a spinning electric motor producing current that back feeds to the distribution bus. Thus, even when the circuit breaker connecting the distribution bus to the main bus is open, current may exist in the distribution bus. In each of the foregoing situations, a properly operating earthing switch can protect technicians and equipment from harm.
Arcing can occur when an earthing switch is closed on a fault. The arcing, in turn, can cause melting of the contact material which can result in welding of the contacts. If the contacts are not opened while the metal is still fluid, a rough surface is produced. The voltage concentrations caused by the spikes on the now rough surface result in an even earlier striking of the arc the next time and can lead to permanent welding of the contacts.
To minimize arcing, many conventional earthing switches include coil springs configured to rapidly close the switch when actuated. Such coil springs are often supported adjacent to the rotatable shaft and operatively coupled to the shaft by a crank arm or other mechanism. When the switch is actuated to close, the spring is configured to act on the crank arm to rapidly rotate the shaft and thereby quickly close the switch.
Current earthing switch designs relying on coil springs are generally bulky since the coil springs and associated mechanisms are supported adjacent the rotating shaft and blade contacts. Further, such prior art earthing switches are not easily scalable to various applications, since most often the blade contacts are welded or otherwise permanently secured to the rotatable shaft. Thus, separate shaft/blade assemblies typically need to be manufactured for different applications.